Question: we showed that you need to expend 8340 btu to...

We showed that you need to expend 8,340 Btu to heat up 20 gallons of water from room temperature at 70F to 120F to produce hot water to take a shower. From this we calculated the amount of natural gas that you would burn to heat 20 gallons of water from room temperature at 70F to 120F to take a shower is 8.34 ft^3. Determine the additional amount of gas that must be burned if the water heater has an efficiency of (a) 80 percent, (b) 85 percent, and (c) 90 percent.

Efficiency = desired output/required output